A coupling of this type is known in which the female coupling element has an internal bore to receive a male element, and a locking member displaceable transversely to the bore against resilient return means, the locking member having a keyhole shaped opening through it in the direction of the bore, the narrow part of this opening normally being held in the bore by the return means and displacement of the locking member bringing the wider part of the keyhole shaped opening into alignment with the bore, so that a male element with a shoulder proximate its free end can be inserted into the bore, the shoulder passing through the wider part of the opening, subsequent return of the locking member by the return means bringing the narrower part of the opening around the male element on the opposite side of the shoulder to the free end to prevent uncoupling. Such a coupling element is referred to hereafter as an element of the kind described. When the elements are coupled the narrower part of the opening acts as a locking fork which is held in place by the resilient return means. Uncoupling is prevented by the annular shoulder of the male element abutting against it. Provided it has an appropriate shape such a locking member can operate equally well whether the annular shoulder on the male element has a flat rear wall transverse to the axis of the male element or a frustoconical rear wall. Such locking members are effective to hold the elements of the coupling in coupled condition in spite of swaying caused by oscillating movement of liquid under pressure which is passing through the coupling.
On the other hand, this arrangement does not give any automaticity in mating the two elements of the coupling. To insert the male element into the female it is necessary for the user to displace the locking member against the action of its return spring to bring the wider part of the keyhole opening into alignment with the bore. To remedy this inconvenience it has been suggested that the female element contains a slidable sleeve mounted to move axially within the female member on the opposite side of the locking member to the mouth of the female element, and which is urged by resilient means into engagement with the wider part of the keyhole opening as the male element is withdrawn in order to hold the locking element with its wider part in register with the coupling axis. Such an arrangement is satisfactory in principle but it considerably complicates the construction of the female element of the coupling and increases the cost price.